I really like /r/povertyfinance though clearly those folks are often going through way, way more trying circumstances I am. I guess I'd be more interested in finances for the alienated precariat.
Because most credit cards offer some sort of cash back/reward for every dollar spent, they are great if you can pay off the balance 100% of the time.
However, if you get hit with interest charges even once, it can wipe out any benefit you gained from the cash back/reward bonus. In such cases those people might be better off using a debit card or other means.
In some cases, they have negative cash flow but don't realize it because they haven't sat down and truly inspected their budgeting and spending. They're constantly deciding which bill they can choose not to pay so that they can still make rent. They think getting a credit card will give them breathing room, so they get a one and slowly the balance creeps up, or it jumps up because they suddenly think they can afford that fancy $2,000 TV.
That being said, if you can handle credit cards, not using them is just leaving money on the table. I get 2% back everywhere (more in certain categories), which is better than most high interest savings accounts. If you don't get that 2% back, you're still paying the interchange fees (~3%) for other people because stores don't charge a different amount for cash/debit vs credit.
However, getting 2% (or more) back isn't worth it if you end up spending more money. The emphasis should be on changing spending behavior, not on payment optimization
My wife isn't very good at saving, so our solution is that I review our spending periodically and we discuss anything over $50 that isn't routine spending (e.g. grocery stores), and we each get $X/month for guilt-free spending. She tends to spend it as she "earns" it, while I save up month to month until there's a good deal so I get more value, add I occasionally treat her if she's run out of cash and has a craving.